Experimental Determination of Tool-Chip Interface Temperature

Department of Mechanical Engineering

NITTTR , CHANDIGARH

Presented By -Deepam GoyalNitesh Parmar

Contents

Introduction

Classification of MethodsCalorimetric MethodDecolorizing AgentTool - Work ThermocoupleMoving ThermocoupleEmbedded ThermocouplePhoto cell techniqueInfra-red photographic technique

Introduction

In many manufacturing processes as well as in tribological applications, it is desirable and often times necessary to have some knowledge on the amount of heat generated and consequent temperature rise (both maximum and average) as well as its distribution in the conduction medium.

Most of the energy expended in plastic deformation and friction in metal cutting and metal forming is converted into heat.

However, the measurement of temperature generally is not such a simple and straight forward matter. The heat partition between two bodies which are in contact and moving with respect to the other is also a difficult problem.

Pioneering work in this area was done by Benjamin Thompson, who in 1798 investigated that the heat generated in the boring of a cannon and developed the concept of mechanical equivalent of heat.

The total heat was measured by performing the drilling operation with the work piece, the chips, and the tool submerged in water.

Three different calorimetric setups were used for determining

(i) the total heat generated in drilling,

(ii) heat in the tool after the cut,

(iii) heat in the chips.

Calorimetric Method

Contd..

The heat in the tool was determined by cutting an sample test bar dry and dropping the tool into the calorimeter immediately upon the completion of cutting. Heat in the chips was obtained by noting the temperature rise of the calorimeter and water into which only chips were permitted to fall.

Contd..

Major points:

Quite simple

Low cost

Inaccurate

Only grand average value

Much of the heat generated in cutting was carried out by the chips ( 70–80%) with 10% entering the work piece, and the remainder into the tool.

Decolorizing Agent

Paints or Tape are placed on the tool or job near cutting point.

Variation of temperature causes change in color.

In steels color of chips may also indicate temperature.

Tool-work Thermocouple

The three laws of the thermoelectricity which are applicable to thermocouples are given by the following :

I. The emf in a thermoelectric circuit depends only on the difference in temperature between the hot junction and the cold junction, and is independent of the gradients in the parts making up the system.

II. The emf generated is independent of the size and resistance of the conductors.

III. If the junction of two metals is at a uniform temperature, the emf generated is not affected by a third metal, which is at the same temperature, used to make the junction between the first two.

In a thermocouple two dissimilar but electrically conductive metals are connected at two junctions. Whenever one of the junctions is heated, the difference in temperature at the hot and cold junctions produce a proportional current which is detected and measured by a milli-voltmeter.

In machining like turning, the tool and the job constitute the two dissimilar metals and the cutting zone functions as the hot junction. Then the average cutting temperature is evaluated from the mV after thorough calibration for establishing the exact relation between mV and the cutting temperature.

Contd..

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Advantages of Thermocouples include the following:

simple in construction,

ease of remote measurement,

flexibility in construction,

simplicity in operation and signal processing,

low cost.

Contd..

Moving Thermocouple Technique

This simple method, schematically shown, enables measure the gradual variation in the temperature of the flowing chip before, during and immediately after its formation.

A bead of standard thermocouple like Chrome-Alumel is brazed on the side surface of the layer to be removed from the work surface and the temperature is attained in terms of mV.

Embedded thermocouple technique

In operations like milling, grinding etc. where the previous methods are not applicable, embedded thermocouple can serve the purpose. The standard thermocouple monitors the job temperature at a certain depth, h from the cutting zone.

The temperature recorded in oscilloscope or strip chart recorder becomes maximum when the thermocouple bead comes nearest (slightly offset) to the grinding zone.

With the progress of grinding the depth, h gradually decreases after each grinding pass and the value of temperature, θ also rises as has been indicated in Fig.

For getting the temperature exactly at the surface i.e., grinding zone, h has to be zero, which is not possible. So the θ vs h curve has to be extrapolated upto h = 0 to get the actual grinding zone temperature.

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m

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The limitations of the embedded thermocouples include the following: difficulty in placing thermocouple close to tool-chip contact & further causes

interference in flow of heat. the technique is difficult to implement as it involves the use of fine holes. thermocouples have limited transient response due to their mass and

distance from the points of intimate contact.plotting of the temperature isotherms using embedded thermocouples in the

tool can be difficult.

Photo-cell technique

This unique technique enables accurate measurement of the temperature along the shear zone and tool flank as can be seen in Fig. The electrical resistance of the cell, like PbS cell, changes when it is exposed to any heat radiation.

The amount of change in the resistance depends upon the temperature of the heat radiating source and is measured in terms of voltage, which is calibrated with the source temperature.

It is evident from Fig. that the cell starts receiving radiation through the small hole only when it enters the shear zone where the hole at the upper end faces a hot surface. Receiving radiation and measurement of temperature continues until the hole passes through the entire shear zone and then the tool flank.

Infra-red Photographic Technique

This modern and powerful method is based on taking infra-red photograph of the hot surfaces of the tool, chip, and/or job and get temperature distribution at those surfaces. Proper calibration is to be done before that. This way the temperature profiles can be recorded in PC as indicated in Fig. The fringe pattern readily changes with the change in any machining parameter which affect cutting temperature.

Contd..

Photographic Technique using IR

References

Bhattacharyya, A, Metal cutting theory and practice, New Central book Agency (P) Ltd. 1984, pp. 360-367.

Juneja, B. L., & Sekhon, G. S., Fundamentals of metal cutting and machine tools, Wiley Eastern Ltd., 1987, (pp. 152-155).

Trent EM, Wright PK. Metal Cutting. 4th ed Oxford, UK: Butterworth Heinemann, 2000.

Abhang L.B., Hameedullah M.: Chip-Tool Interface Temperature Prediction Model for Turning Process, International Journal of Engineering Science and Technology, Vol 2(4), 2010., pp. 382-393.

www.nptel.iitk.ac.in

E-mail: er.nitesh41@gmail.combkdeepamgoyal@gmail.com